1) Field of the Invention
The present invention relates to a signal light processing apparatus and more particularly to a signal light processing apparatus suitable for use in a receiving system in an optical communication system.
2) Description of the Related Art
In a general optical communication system, an optical receiving apparatus 100 shown in FIG. 15, for example, receives signal light transmitted through an optical fiber, which is a transmission path. In the optical receiving apparatus 100, a photoelectric converter 101 such as an APD (Avalanche Photo Diode) or the like receives the signal light transmitted through the transmission path and converts the signal light into an electric current signal. A preamplifier 102 converts the electric current signal into a voltage signal and amplifies the voltage signal. A slicing amplifier 103 further amplifies the voltage signal, and outputs the voltage signal to both a flip flop 104 for regenerating a data signal (DATA) and a clock extractor 105 for regenerating a clock signal (CLK).
At this time, the slicing amplifier 103 can convert the voltage signal fed from the preamplifier 102 into a voltage signal of a rectangular wave according to the magnitude of a predetermined discrimination level generated by a discrimination level generating source 103A, and output it to the flip flop (F/F) 104 and the clock extractor 105. Whereby, the slicing amplifier 103 can practically detect a binary signal (main signal data) modulated on the signal light as the voltage signal of the rectangular wave.
In recent optical communication systems such as WDM transmission systems, OSNR (Optical Signal to Noise Ratio) of signal light inputted to the optical receiving apparatus 100 largely changes according to the transmission distance, system configuration and functions. For the discrimination level generated by the discrimination level generating source 103A, an optimum value is determined according to the OSNR in an optical communication system in which the optical receiving apparatus 100 is disposed.
For instance, the optical receiving apparatus 100 in an optical communication system having a relatively small number of stages of optical amplifiers interposed on the transmission path receives signal light having such characteristics that the noise is relatively small (that is, the S/N is excellent) but the receiving level is relatively small. Particularly, the optical receiving apparatus 100 in a minimum receiving system in which no optical amplifier is interposed receives signal light at an extremely small receiving level.
FIG. 17(a) shows an example of eye patterns of such received signal light, and FIG. 17(b) shows BER (or S/N) characteristic of the signal light according to the discrimination level setting value. As shown in FIG. 17(b), when signal light having small noise is received, the S/N is excellent if the input level is relatively large, the V-characteristic of the discrimination level setting curve is steep, a range of the valley of the V being wide and deep.
Accordingly, a settable range of the discrimination level satisfying BER (specified BER) specified by ITU-T (International Telecommunication Union Telecommunication Standardization sector) can be set within a relatively wide range between an optical level at which data of “1” has been modulated and an optical level at which data of “0” has been modulated.
However, when the receiving level is smaller, a difference between the optical levels at which data of “1” and “0” have been modulated is smaller, which leads to a narrower eye opening due to noise appearing in the vicinity of these levels. Thus, it becomes necessary to accurately adjust the setting of the data discrimination level. Since noise components appearing in the vicinity of an optical level at which data of “1” has been modulated are generally at a larger level than noise components appearing in the vicinity of an optical level at which data of “0” has been modulated, the discrimination level is required to be offset below the intermediate point between the above “1” level and “0” level, that is, below a 50-percent level.
In this case, when the average optical input power is 10 dBm, for example, the input amplitude of the slicing amplifier is 100 mVpp. And, when the discrimination level voltage is set at a 40-percent level of a difference between the above levels, it is sufficient to give an offset of 100 mV, for example, from the 50-percent level. If the above average optical input power is so small as 20 dBm, it is necessary to give an offset of 10 mV, for example, which is a relatively high-accurate offset, from the 50 percent level when the discrimination level voltage is set at a level of 40 percent like the above case.
To the contrary, the optical receiving apparatus 100 in an optical communication system in which a relatively large number of stages of optical amplifiers are disposed on the transmission path, that is, a maximum receiving system, receives signal light having such characteristics that the noise is relatively large due to ASE light and the like generated in the optical amplifiers (that is, the S/N is not excellent) and the receiving level is relatively large. FIG. 18(a) shows an example of eye patterns of such received signal light, and FIG. 18(b) shows a BER (or S/N) characteristic of the signal light according to the discrimination level set value.
As shown in FIG. 18(b), when signal light inputted through a transmission path having a large number of stages of optical amplifiers interposed, the S/N is deteriorated, thus the V-characteristic of the discrimination level set curve is smooth, which leads to a shallow region of the V. Since the receiving level is relatively large, the optical levels at which data of “1” and “0” have been modulated are large, but the noise levels appearing in the vicinity of the optical levels of “1” and “0” are large, as well (because the V-characteristic itself is deteriorated). Accordingly, a settable range of the discrimination level satisfying the specified BER is estimated to be narrower than the case shown in FIG. 17(b) (within a range based on the ratio).
As above, an optimum value of the discrimination level is set according to OSNR in an optical communication system in which the optical receiving apparatus 100 is disposed. In order to provide the optical receiving apparatus 100 that can be mounted in various types of communication system, setting of the discrimination level generated by the discrimination level generating source 103A is provided in menu for each type of the apparatus, and a relatively large number of types of the apparatus having various discrimination levels are prepared in the product menu when products of the optical receiving apparatus are provided. However, such preparation of a relatively large number of variations of the setting of the discrimination level causes an increase in developing cost and management cost due to an increase in types of the apparatus.
If various setting of the discrimination level in the optical receiving apparatus is prepared in order that excellent discrimination level can be set in both the minimum receiving system and the maximum receiving system, an increase in cost due to such an increase in type becomes a more serious problem.
From the above drawbacks, it is desired to monitor the S/N of signal light inputted to the optical receiving apparatus to control the discrimination level, whereby the number of types of the setting of the discrimination level is decreased while the discrimination level is set optimally regardless of whether the optical receiving apparatus is disposed in the minimum receiving system or in the maximum receiving system to realize a general-purpose apparatus.
FIG. 16 is a block diagram showing an optical receiving apparatus 110 to which a structure controls the discrimination level is applied. In the optical receiving apparatus 110 shown in FIG. 16, the part consisting of the slicing amplifier 103, the discrimination level generating source 103A, the flip flop 104 and the clock detecting unit 105 is shown as a discriminator 113.
In the optical receiving apparatus 110, a DEMUX 106 converts a signal whose data has been regenerated by the flip flop 104 configuring the discriminator 113 into parallel data in N columns, and an FEC (Forward Error Correction) 107 performs error correction on the parallel data. An error information extracting unit 108 extracts information about the number of times the error correction has been performed by the FEC 107, and a discrimination level operation controlling unit 109 operates and controls the discrimination level to be used in the discriminator 113, by using the error information extracted by the error information extracting unit 108 as parameter.
Such error correction by the FEC 107 relieves BER required in the discrimination stage of the received signal light as compared with a case where the error correction by the FEC 107 is not performed. In other words, in order to satisfy the specified BER, the level of BER specified for discrimination by the discriminator 113 is relieved as compared with a case where the error correction is not performed in the later stage. Particularly, it is possible to widen the setting range of the discrimination level when the discriminator 113 performs discrimination on received signal light transmitted through a transmission path on which an optical amplifier is interposed [refer to FIGS. 18(a) and 18(b)].
Meanwhile, as to an optical receiving apparatus in an optical communication system, the transmission distance (that is, a length of the transmission path fiber between regenerative repeaters) is limited due to deterioration of the received waveform caused by the dispersion characteristic of the transmission path fiber. For example, signal light having an eye pattern shown in FIG. 19(a) before the signal light is transmitted is deteriorated to one shown in FIG. 19(b) or 19(c), for example, because of the dispersion characteristic of the transmission path fiber.
It has been examined to provide a function of compensating received waveform deterioration caused by the dispersion characteristic of the transmission path fiber to the optical transmission apparatus on the receiving side. By providing the function of compensating the received waveform deterioration, it is possible to improve the functions of the optical transmission apparatus and increase the length of the transmission path fiber.
FIGS. 20(a) through 20(c) are diagrams showing examples where there are given to respective optical transmission apparatuses 122A to 122C on the receiving side functions of compensating the received waveform deterioration when the optical transmission apparatuses 122A to 122C receive signal light outputted from transmission units 121a in optical transmission apparatuses 121 on the transmitting side in optical communication systems, in which the optical transmission apparatuses 121 on the transmitting side are connected to the optical transmission apparatuses 122A to 122C through transmission path fibers 120, respectively.
In the optical transmission apparatus 122A shown in FIG. 20(a), a dispersion compensating fiber 122c is interposed between an optical amplifier 122a and an optical receiving unit 122b. In the optical transmission apparatus 122B shown in FIG. 20(b), an optical dispersion compensator 122d is interposed between the optical amplifier 122a and the optical receiving unit 122b. In the optical transmission apparatus 122C shown in FIG. 20(c), an electric dispersion compensator 122b-3 is disposed between a light receiving unit 122b-1 and a discriminating/regenerating unit 122b-2. The light receiving unit 122b-1, the discriminating/regenerating unit 122b-2 and the electric dispersion compensator 122b-3 are forming an optical receiving unit 122b′ in the following stage of the optical amplifier 122a. 
As known techniques relating to this invention, there are techniques described in following patent documents 1 through 4:
[Patent Document 1] Japanese Patent Application Laid-Open Publication No. H11-346194
[Patent Document 2] Japanese Patent Application Laid-Open Publication No. 2003-304202
[Patent Document 3] Japanese Patent Application Laid-Open Publication No. 2004-15587
[Patent Document 4] Japanese Patent Application Laid-Open Publication No. H09-181687
As to the apparatus shown in FIG. 16, since it is necessary to encode the main signal with a unique FEC code in both the optical transmitting apparatus and the optical receiving apparatus, the apparatus shown in FIG. 16 is difficult to be disposed opposite to an apparatus without the corresponding FEC encoding/decoding function. Therefore, a problem may be encountered when the apparatus is connected to an existing apparatus or a different system vendor.
To overcome this problem, it is possible to integrate a device having the FEC function in the optical module to enable connection of the apparatus to an existing apparatus by only exchanging the parts of the apparatus. However, the size of an integrated circuit having the FEC function is considerable, which leads to an increase in size of the optical module when such integrated circuit device is integrated in the optical module.
When the dispersion compensating fiber 122c is interposed to compensate waveform deterioration as shown in FIG. 20(a), it is necessary to prepare a menu of the dispersion compensating fiber corresponding to the length of the transmission path fiber to cope with the dispersion characteristic that differs according to the length of the transmission path fiber, which leads to an increase in the manufacturing cost when the apparatus is produced.
When the optical dispersion compensator 122d is interposed as shown in FIG. 20(b), it is difficult to automatically set the compensation amount because the degree of waveform deterioration is difficult to be detected. When the electric dispersion compensator 122b-3 is provided in the optical receiving unit 122b′ as shown in FIG. 20(c), the control for compensating the amount of waveform deterioration is unstable because the waveform deterioration is compensated on the basis of a signal containing both the waveform deterioration amount and the noise components.
The inventions described in the Patent Documents 1 through 4 fail to provide a technique that can attain optimum setting of the discrimination level regardless of whether the optical communication system is in the minimum receiving system or the maximum receiving system, and can extract the amount of waveform deterioration from a received signal waveform.